The method is based on the comparison of local vapor concentrations at the inlet and outlet refrigeration ports and taking action based on that comparison. Control apparatus is installed inside a refrigeration enclosure (10), adjacent to a port (14,15), preferably at the lowermost port (14). If the enclosure (10) contains multiple ports (201,202) at similar height, then each port (201,202) has a form of the control apparatus attached to it. The control apparatus adjusts a flow of vapor leaving the interior of the enclosure (10). The control apparatus includes a duct assembly (17,214,224,300,401,501,601) and a blower system (26,210,220,403,503,603). The bottom portion of the duct assembly (17,214,224,300,401,501,601) is a tunnel enclosure (20,22) through which a conveyor belt (16,203) passes. Connected to an inside edge of the tunnel enclosure is a duct (23) that extends upward from the conveyor belt (16,203). A blower (26,210,220,403,503,603) for this duct (23) either sucks vapor away from the conveyor belt (16,203) or blows vapor from the enclosure interior (12) toward the belt (16,203). Regardless of the flow direction, a vapor curtain forms inside the tunnel enclosure and represents a transitional region from all vapor to all air. Control of the blower (26,210,220,403,503,603) for the duct assembly is based on vapor concentrations in the tunnel enclosures (20,22) adjacent to each port (14,15,201,202). A microprocessor compares measured concentration levels and alters the blower motor (27,211,221,404,504,604) frequency in such a manner as to minimize the difference in concentration levels at each port (14,15,201,202).